Propagate system identifier generation improvement into pg_resetxlog.

Commit 5035701e improved xlog.c's method
for creating a database system identifier, but I neglected to fix the
copy of that code appearing in pg_resetxlog.c.  Spotted by Andres Freund.

src/bin/pg_resetxlog/pg_resetxlog.c

@@ -492,7 +492,8 @@ GuessControlValues(void)
 	 */
 	gettimeofday(&tv, NULL);
 	sysidentifier = ((uint64) tv.tv_sec) << 32;
-	sysidentifier |= (uint32) (tv.tv_sec | tv.tv_usec);
+	sysidentifier |= ((uint64) tv.tv_usec) << 12;
+	sysidentifier |= getpid() & 0xFFF;
 
 	ControlFile.system_identifier = sysidentifier;